Monoclonal antibodies (MAbs) have been produced to F1 (low-metastatic variant) and F10 (high-metastatic variant) lung metastatic variants of the C57BL/6J mouse B16 melanoma, using spleen cells from tumor-bearing mice. Three MAbs are currently being investigated. 3E9 IgG MAbs, produced to B16-F10 cells, cause tumor cell clumping and have complement-dependent cytotoxic activity in vitro. By indirect immunofluorescence, 3E9 reacts with a membrane-associated antigen that shows capping and antigen extrusion in ethanol-fixed cells. 3E9 reacts with F1 and F10 cells, Lewis lung tumor cells, SV3T3 cells and LM cells, but not with C57BL/6J thymocytes or 3T3 cells. 3C10 IgM MAbs and 1F4 IgG1 MAbs, produced to B16-F1 cells, each react with a membrane antigen on B16-F1 and F10 cells that does not show antigen extrusion, and with LM cells and C57BL/6J thymocytes. By SDS-PAGE and autoradiography of reduced immunoprecipitates formed with 125I-labeled F10 membrane extract, the 3E9 and 3C10 antigens have minimal molecular weights of 25 and 40 kilodaltons, respectively. Groups of 10 C57BL/6J mice treated with 3E9 or 3C10 MAbs and injected with F1 or F10 cells show altered lung colonization compared with untreated controls. Both antibodies cause a two-\to three-fold reduction in F10 tumor colonies and a two-\to three-fold increase in F1 tumor colonies (P equal to or less than 0.05). Experiments are in progress to test a novel immunotherapy model, using tumor-bound MAb as a protein antigen to which cell-mediated immunity will be selectively induced.